The study of biological samples with the aid of a phase contrast has long been known in light microscopy. Using a so-called phase ring, which is situated in the back focal plane of a light microscope's objective, the radiation coming from a sample to be studied, which contains diffracted and undiffracted components, becomes phase-shifted by 90° (i.e., by π/2) in the zeroth order of diffraction. The interference of the higher orders of diffraction with the phase-shifted beam delivers a contrast-rich image (image having a phase contrast).
The use of a method of this type in electron microscopy, in particular in transmission electron microscopy, has for long not been possible. However, even in that case the use of a phase contrast study is advantageous because biological samples are basically very thin and have mostly very light-weight atoms, so that they are almost transparent for incident electrons/electron waves. Samples of this type are known as weak-phase objects. Instead of modifying the amplitude of an incident electron wave, weak-phase objects cause only a phase shift of the incident electron wave. To obtain a better phase contrast, an additional 90° (i.e., π/2) phase shift between the scattered or diffracted and the unscattered or undiffracted electrons is needed.
Some proposals have already been made to achieve a phase contrast in transmission electron microscopy.
On the one hand, it has been proposed to achieve the phase contrast in a transmission electron microscope (hereinafter referred to as TEM), which has an objective lens, by defocusing the objective lens and the spherical aberration of the objective lens. It is, however, disadvantageous here that only a spatial frequency-dependent phase shift is achieved via this procedure, so that the achieved phase contrast is also spatial frequency-dependent. Samples to be studied having very low spatial frequencies have, therefore, a very low contrast. This relationship may be described in greater detail using the phase contrast transfer function (PCTF or CTF for short) (see, for example, Optimizing phase contrast in transmission electron microscopy with an electrostatic (Boersch) phase plate, E. Majorovits et al, Ultramicroscopy 107 (2007) 213-226).
On the other hand, it has been proposed to use a so-called Zernike phase plate which provides a uniform phase shift of scattered or unscattered electrons for all spatial frequencies. A phase plate of this type is made of a thin carbon film, for example, which is provided with a small hole. This carbon film is situated in the back focal plane of an objective lens of a TEM. Unscattered electrons pass through the small hole of the carbon film, while scattered electrons hit the carbon film itself and may suffer a phase shift of 90° (π/2) due to a homogeneous isotropic potential (internal potential, Coulomb potential) of the carbon film. It has been found, however, that the carbon film is rapidly contaminated. In particular the small hole is easily clogged. Furthermore, it is disadvantageous that the carbon film is rapidly charged.
Furthermore, a so-called Hilbert phase plate which, like the Zernike phase plate, may also be made of a thin carbon film and causes the phase shift of a half-space, is also used for phase shifting. However, the Hilbert phase plate has the same disadvantages as the Zernike phase plate.
An alternative form of the phase shift in a TEM is achieved by using a Boersch phase plate in which an electron beam is exposed to an electrostatic potential. A design of such a Boersch phase plate is known, for example, from EP 782 170 A2. It has a ring electrode and a holding device. The ring electrode is made of an annular plate having a central opening and an outer edge. The holding device is formed by two straight supports which are situated opposite each other. In this known Boersch phase plate, the electrons unscattered or undiffracted on a sample pass through the central opening, while the scattered or diffracted electrons pass outside the annular plate. An electrostatic field which causes a phase shift of the electrons passing through the central opening is formed in the central opening. However, the known Boersch phase plate has the disadvantage that, in particular due to the support, partial beams are shadowed and prevented from contributing to the interference, so that information is lost.
Another Boersch phase plate is known from WO 03/068399 A2. This document relates to a phase plate having a ring electrode which is designed as a plate having an opening and a peripheral outer edge. Furthermore, a holding device including supports is provided. The supports are applied to the outer edge of the ring electrode and are used for positioning the ring electrode in a TEM. This known phase plate is characterized in that the holding device does not have a central symmetric design and no further support is situated on the outer edge of the plate opposite to each support with respect to the center of the opening. The known phase plate is situated in the back focal plane of an objective of a TEM. Although, due to the supports of the phase plate, a partial area of the scattered electrons is shadowed in the back focal plane of the objective, due to the non-centrally symmetrical arrangement, the scattered electrodes remain unaffected in a further partial area which is centrally symmetrical to the shadowed partial area. Due to the symmetry relationships, this phase plate allows the image information initially lost due to the shadowing by the supports to be reconstructed. The disadvantage of this known phase plate is, however, also that a partial area of the scattered electrodes is shadowed and therefore part of the image information must be reconstructed.
DE 102 00 645 A1 describes an electron microscope which also has a phase-shifting element. This phase-shifting element is ring-shaped and has a central opening, and it is therefore able to be secured at its outer periphery, so that no cantilever or almost-cantilever structures are needed.
Accordingly, it would be desirable to provide a phase-shifting element and a particle beam device having a phase-shifting element, in which components shadowing the particle beam are avoided, so that proper information content is achieved and in which the phase contrast is essentially spatial frequency-independent at zero defocus (Gauss focus).